1. Field of the Invention
This relates generally to internal combustion engine ignition systems and in particular, to an automotive ignition system that generates a constant ignition coil control signal over the normal range of operation of the engine.
2. Description of the Prior Art
An ignition system is utilized in internal combustion engines to provide ignition sparking potential to the spark plugs. The conventional ignition system commonly used today uses breaker points to periodically interrupt a current in the primary of an ignition coil. When the points are separated by, for example, the distributor camshaft, the current then flowing through the primary of the ignition coil decreases rapidly, inducing a high voltage in the coil secondary which is selectively connected via the distributor to an appropriate spark plug. At the spark plug, the high induced voltage ionizes the gap between electrodes which causes a compressed air/fuel mixture associated with the cylinder to ignite.
Although widely used, the conventional system is subject to variations in spark intensity and timing over the system lifetime due to combinations of the following factors: unpredictability of the rate of decrease of current in the primary due to variations in the arc formed at the contact regions of the points; changes in the current in the coil just before it is interrupted due to changes in resistance when the breaker points are closed; changes in ignition timing accuracy relative to crankshaft position due to mechanical wear of the cam follower and other components; changes in signal due to "point bounce" which is a form of harmonic motion set up by the points under certain speed/acceleration conditions; changes in the mechanical and electrical characteristics of the breaker points due to electrical, chemical, and mechanical stress; and wide variations in coil primary current due to changes in vehicle battery voltage, the series resistances in the coil primary current path, the ambient temperature, and the resistance of the breaker points.
Each of these factors affect the peak output voltage of the coil, and consequently the voltage presented to the spark plug. As a result the sensitivity of the system to changes in spark plug gap ionization potential is increased over its lifetime, which, in turn, causes a decrease in the effective spark plug life. It has been found that as plugs age, their tendency to misfire increases. Misfire adversely affects performance, economy and emissions output of the engine.
One type of automotive ignition system is limited to using a distributor magnetic pickup voltage waveform to establish on and off times of current through the primary winding of the ignition coil. To compensate for cycle-to-cycle error in the continuously varying pickup voltage, the system employs a feedback signal which is generated during one cycle of operation and used to control current generation in succeeding cycles. However, the system does not provide the degree of control desired with magnetic pickups which exhibit cycle-to-cycle errors.
Other types of ignition systems are found in U.S. Pat. No. 3,238,416 issued Mar. 1, 1966, and entitled "Semiconductor Ignition System" by G. D. Huntzinger et al., U.S. Pat. No. 3,487,822, issued Jan. 6, 1970, and entitled "Capacitor Discharge Ignition System", by A. G. Hufton et al., U.S. Pat. No. 3,605,713, issued Sept. 20, 1971, and entitled "Internal Combustion Engine Ignition System", by P. D. LeMasters et al., and in copending U.S. continuation patent application Ser. No. 419,172, filed Nov. 26, 1973, and entitled "Automotive Ignition Control," by A. A. Adamian et al.